I know this one's impossible, but Punch-Out x God of War. Little Mac taking on skeleton soldiers, satyrs, cyclopses, the gods, titans, and of course Kratos. It'd be like that Perry Bible Fellowship comic where he's ready to take on the world.

Besides that Sega has to revive Amusement Vision and let them make another F-Zero again.
Another Mario RPG from Square would be interesting and while we're at it: A Codemasters Monster Game collaboration.